Communication and control system for airplanes



Sept. 23, 1947.

y A..J. soRENsEN COMMUNICATION AND CONTROL SYTEM FOR AIRPLANES 2 .Sheets-Sheet 1 Filed Sept. 4. 1943 Sept# 23, 1947. A. J. soRENsEN COMMUNICATION AND CONTROL SYSTEM FOR AIRPLANES Filed sept. 4. 194s 2 Sheets-Sheet 2 Patented Sept. 23, 1947 COMMUNICATION AND CONTROL SYSTEM FOR AIRPLAN ES Andrew-J. Sorensen, Edgewood, Pa., assignor to TliezUnionSwitch & Signal Company, SwissvalePa., a corporation of Pennsylvaniar` Application' September 4, 1943, Serial No; 50i-,2.74

2 Claims.

My invention relates to communication and control systems for air planes, and more particularly to apparatus/forcontrol andY communication systems for' tractor planes and gliders.

When gliders or motorl'ess planes are pulled by a tractor or tow plane; the glider pilot has difficulty in maintaining proper direction and position relative" t'olthe tractor plane in taking oii and landing, anrlI also during periods of low visibility while in: flight. If proper direction and position of a gliderisi-not maintained, then there is the possibility that the glider may cause the tractor planev to stall' and" cause a forced' landing of both planes. WhenV twoA or three gliders are towed ,by a single tractor plane these conditions become more acute. An` intercommunication and control system installed between the tractor plane andi' the glider' would airv greatly the pilots in keeping the planes in proper relative positions and. direction. IiA an "automatic pilot is usedon af glider, av reliable control system by which tl`1et'ractor` planev pilot controls the glider is requiredi Accordingly, a feature of'my invention is the provision of novel' and improved apparatus for communication and control systems for air planes.

Another feature ofli my-v invention is the provision of novel and' improved apparatus for communication and control'- systems for tractor plane and gliders pulledthereby and wherewith privacy of communication and reliability of controlv are effected.

Again, a feature of my invention is the provision of novel means f'or communication and control between a tractor plane and a glider through the towing cable;

Still another feature of` myl invention is the provision of novel apparatus for a pilot of a tractor plane to automatically establish different controls for one or more gliders being towed by that tractorA plane.

Other features, objects and advantages of my invention will lappear as the specication progresses.

The above features, objects and advantages of my invention I accomplish byprovlding communication and controliapparatus that is mountedv partly on eachI plane, and forming a circuit between the two portions of' the apparatus through the towingfcable andthe capacitance between the metal portions of the two planes. A towing cable mustz-be' capable of expanding and contractingA itsr length somev thirty per cent or more becauseof the-natureof the task, and for this reason the cable is` made usually of' nylon and otherV elastic material'which are electrically non-conductive. ToA provide'the cable with electric conductive properties, I'impregnate or other- Wise treat the cable withr some suitable cornpound, such as, for'exampleaquadag. A few pounds of suchv line carbon particles found in aquadag will make such a cable of the length of 500 feet, which has' been' found suitable for towing gliders by tractori planesconductive to the extent that its resistance is` of the order of 10,000 ohms. In otherwords, the towing cable lis treated. to providewithin reasonable limits conductive properties of a` predetermined value. This treatment of a cable may be effected either during the manufacturingy process and its conductivity made substantially uniform over its entire length, or it. may be treated after it has been cut to proper length andthe treatment confined to a preselected portion between the two connections to the planes, leaving a short non-conductive length. of cable adjacent each plane, which short length willr serve` as an insulator between the planel and the conductive portion of the cable, When the entire cable is treated, then insulators of thestrain insulator type are inserted in the cable, one adjacent its connection to each" plane. in eitherv case the body of each plane is electrically insulated from the intermediate treated portion of the towing cable.

The metal'bodies and other metal portions of the two planes form ay condenser the dielectric of which` is the airspace between the planes. Also, the metal body of. each plane is one plate of a condenser, the other plate: of which is the earth. Thus with the towing cable treated as explained above and insulatedfromzthe bodies of the planes, a transmitting circuit is formed between the two planes, one side of such circuit including the treated portion of the towing cableand the other side including the capacitance between the metallic portions of the planes in multiple with the capacitance between the metallic portion of each plane and earth and the ground path.

The communication and control apparatus is connected to this transmitting circuit by the portion on each plane being connected between a terminal secured to the treated portion of the towing cable outside of the adjacent insulator, anda terminalformed on the metalportion of the respective plane.

The apparatus for communication and control includes transmitters and receivers, a transmitter mounted on one plane cooperating with a receiver on the other plane. Each such transmitter includes a source of carrier Wave and means to modulate such carrier by a modulating wave, and each receiver includes a demodulator of such modulated carrier and means responsive to the modulating wave. I propose to use a carrier Wave of the frequency of the order of 10,000 cycles per second or less so that it is substantially free from electromagnetic radiation but is effective for electrostatic induction. Consequently, when a transmitter on one plane and the receiver on the other plane are connected to this transmitting circuit formed between the two planes, energy is transferred between the two portions of the apparatus, that is, between the transmitter and the receiver, due to the conductive properties provided for the towing cable and the capacitance between the metallic portions of the planes and the capacitance between the metallic portion of each plane and earth. In other words, a transfer of energy between a transmitter and a receiver takes place because of the potential difference that is created between the metallic portions of the planes. In case either plane is constructed of non-metallic material, such as plywood, I insert a thin metal sheet in the wings or in some other suitable part of the plane and connect the transmitting circuit to this plate. For communication, the carrier wave is modulated by a voice current, and the receiver includes a loud speaker or some other form of telephone receiver. For control the carrier is modulated by different preselected frequencies an individual frequency being used for each control. For twoway communication and control each plane is provided with a transmitter and a receiver, together with switching means for connecting either the transmitter or the receiver of a plane to the transmitting circuit. When more than one glider is being towed, the apparatus on the tractor plane is connected to the cables to the different gliders in multiple, and all the pilots are in communication with the tractor plane pilot and orders can be given to keep the proper relative positions of the planes with respect to each other and with respect to the tractor plane. Normally each apparatus is connected to the transmitting circuit in a receiving condition, and communication can be initated by any one of the pilots. If communication and control are both provided, each equipment is provided with switching means having different positions to selectively connect the respective portion of the apparatus at its different control and communication conditions to the transmitting circuit.

I shall describe two forms of apparatus embodying my invention, and shall then point out the novel features thereof in claims.

In the accompanying drawings, Fig. 1 is a diagrammatic view showing one form of apparatus embodying my invention when used for telephone communication between a tractor plane and a glider. Fig. 2 is a diagrammatic view showing a form of apparatus embodying my invention when used for a combined control and telephone communication between a tractor plane and a glider.

In each of the two views, like reference characters are used to designate similar parts.

Referring to Fig. 1, the reference characters TP and GL designate a tractor plane and a glider, respectively, the tractor plane TP pulling glider GL through a towing cable TC connected to each plane in any suitable manner. This towing cable TC would be of suitable size and length and a length of 500 feet has been proposed. The cable is, ordinarily at least, made from nylon or other synthetic material because it must be capable of expanding and contracting in length some thirty per cent or more. Since such material makes the cable electrically non-conductive, the cable TC is treated with a conductive compound, such as, for example, aquadag either during the manufacturing of the cable or after the cable has been prepared ready for connection to the planes. By such treatment the cable is made to have a conductive property and a length of the order of 500 feet may have a resistance of the order of 10,000 ohms. Assuming that the cable TC has been treated throughout its entire length, two insulators 4 and 5 are interposed in the cable adjacent its connection to the planes TP and GL, respectively. In case the cable is treated after it has been made ready for connection to the plane, then a short length of the cable adjacent each plane may be left untreated, and the insulators 4 and 5 may not be required because the untreated portion of the cable would serve as an insulator. The metallic'bodies and other metallic portions of the planes form the plates of a condenser the dielectric of which is the air space between these two planes. In order to visualize such capacitance dotted lines I5 and I6 and a condenser I "I are added to the drawing, although they represent no additional physical apparatus. Also, the metallic body and other parts of each plane form one plate of a condenser of which the other plate is the earth and the dielectric is the air space. To visualize such capacitance dotted line condensers I8 and I9, together with respective dotted line connections, have been added to the drawing, but it is to be understood that these condensers and connections represent no additional physical apparatus. It is clear that a circuit is formed between the two planes and of which circuit the treated portion of the cable TC intermediate insulators 4 and 5 forms one side ofy the circuit and the capacitance represented by condenser Il in multiple with the capacitance represented by condensers I8 and I 9 and the ground path forms the other side of the circuit. As stated hereinbefore, if either plane is made of non-metallic material, then a light metal sheet would be inserted in the plane wing or some other suitable part.

Planes TP and GL are provided with communication apparatus mounted partly on each plane. In Fig. 1, two-way communication is contemplated and the two portions of the apparatus are dup-lications and each includes a transmitter and a receiver.

Looking at plane TP, the transmitter includes a modulator-amplifier MAI, a microphone MI, and an output transformer TI. The modulatoramplifier MAI may be any one of several wellknown forms, and is shown conventionally since the construction of this device forms no part of my invention. For example, the modulatoramplier may be similarto that covered in Letters Patent of the United States No. 2,287,655, granted June 23, 1942, to L. D. Whitelock et al., for Transmitting apparatus for communication systems. It is suflicient for a full understanding of my invention to point out that within the modulator MAI, a carrier wave is generated and modulated by a voice current created by speaking into the microphone MI, microphone MI being connected to the modulator through a battery `.output transformer TfI toiinduce ya :corresponding Ielectromotive force .fin lsecondary lwinding 18 :of

that transformer. `Secondary'winding 8of .trans- .former TI is connected tothe transmitting cir- :cuit extending between the two .planes vby one lterminal 'of 'winding .'8 being connected to a ter- .minal i9 securedto the towing cable TC through,

`wire IILnormallyopen-'contact II of 'a manually operable switch SWI vand wire I2; fand the other "terminal tof -winding ".8 `being vconnected to a `terfminal I3 .formed in 'the metal 'portion oi `the .bodyfofxtheplanefTP throughwire V4. It follows that 4when .switch SWI .is operated .to the dotted line position Aclosing vcontact .II.,' vthe carrier telephone current created through the modulatoramplier 'MAI is impressed .between the terminals 9 and :I3 of "thetransmitting circuit, 'that zis, `a vcorresponding potential is created between the-conductive .portion ofthe towing-cable .and the `metal part of the plane.

"The receiver of the apparatus on plane TP :includes a rst stage'ampliertube VI, a demodu- -lator-'amplifierjDAI and-a loud speaker LSI. The tube -`VImayzbeof any `suitable type `of electron tube, and inFig. 1 it 'is an indirectly heated tube 'having ananode`20, a control lgrid 12|, a cathode r:22 and a filament l23, the `lament 123 being con- :stantly 'heated .in any suitable arrangement. A grid ycircuit for .tube VI includes abias unit 24 Aand a vresistor '25,rthe resistor 25 having connections between terminal 9 of `cableTC and terminal I3 of the body ofthe plane through a normally fclosed 4:contact 2li of switch SWI Ias will be ap- #parent by .an .inspection of Ythe drawing. The anode circuit includes abattery 21 and a rresistor 2B. It is clear that if a carrier telephone current, :such as, for example, that supplied 'by the associatedtransmitter of plane TP be made to appear :across terminals 9 and I3 of the transmitting ciracuit'while contact 26 of switch SWI is closed, such current is impressed on the grid circuit of the itube Vl. and madezto appear in the anode circuit of that `tube inan amplified form.

V'Ihe demodulator-ampliiier DAI may be of any one of several well-known arrangements, and is shown conventionally for the sake of simplicity. 'This device may be similar to the balance type .covered by Letters Patent of the United States No. 2,129,313, :granted September 6, 1938, 'to L. D. Whitelock, for Apparatus for electric `signaling system. It is sufficient for this applica- .tion to point `out that a carrier wave generated within the demodulator is mixed with the incoming carrier telephone current applied yto the 'input sideof thedevice -through wires '29 Vand 30 and a voice current is 'suppliedto the loud speaker LS connected to 'the-output side of the ydemodulator-amplier DAI through wires l3I and 32. The input side of thedemodulator is coupled to the anode circuit of the tubeVI through a condenser 33, yand consequently, the speech of a Ycarrier telephone current appearing at terminals 9 and I3 of the transmitting rcircuit is reproduced at the loud Aspeaker LSI That portion of the apparatus mountedon the glider GL 'includes .a transmitter and a receiver which are substantially the same as the corresponding devices of the tractor plane, and further description lis not required except to point out that the glider transmitter includesa microphone M2, axmodulator-ampliiier `MA2 andan output 'transformer T2; and the glider receiver includes la rst stage amplifying tube V2, a demodula'toramplier DA-2 and .aloud'speaker L82. Also, secondary winding `3l! of outputA transformer T2 'is connected to the transmitting circuit by one rterminal of winding /34 `being connected to 4terminal 35 formed on Vthe towing cable through Wire 36, normally open contact v31 of a manually operable switch SW2 and wire '38; and its other terminal being connected to terminal 39 formed on the metal body of glider 'GL through wire 40. Furthermore, resistor 4I included in the grid circuit for tube V2 is connected across terminals 35 and 39 vthrough a normally closed Contact 42 of switch SW2, as will Vbe apparent on an inspective of the drawing.

Normally, that iis, when the two switches SWI and SW2 are set at the positionsl illustrated bythe solid lines in Fig. 1, the receiver of each plane is connected to the Atransmitting circuit ready .to receive a telephone message. If the operator of the tractor plane TP wishes tofspeak to the pilotof the glider GL, the procedure may be as follows: The tractor plane pilot moves switch SWI to the dotted line position and `speaks some designated calling phrase into the microphone MI. `Such action causes a corresponding carrier telephone current to be impressed 'between terminals 9 and I3 of the transmitting circuit, and a corresponding electromotive force to appear between terminals 35 and 39 at the glider due to the transmitting circuit formed between the two planes. Such electromotive force appearing at terminals y35 and .39 is applied totube V2 and thenceto thefdem'odulator-amplifier and after demodulation the calling phrase is sounded at loud speaker LSE. Alter sending such .calling phrases the` tractor plane pilot would place switc'h SWI at vits normal position. Upon `receipt of such calling phrase the lglider pilot would move his switch SW2 to the dotted line kposition and repeat an agreed acknowledging phrase into his microphone M2. This lacknowledging phrase modulates the carrier at the modulator-,ampliiier MAZ vand a corresponding carrier telephone current is impressed across terminals -35 and 39 of the transmitting circuit, and in turn a corresponding electromotive force is made to appear at terminals 9 and I3,

lwhich'electromotive force is now applied 'to Vtube VI fandin turn to the demodulatorm amplier DAI `with the result that the acknowledging phrase is sounded at loud speaker LSI. From this point -on the `two pilots would carry on a conversation :back and forth by proper positioning of their switches SWI and SW2.

.It is apparent that if the glider .pilot to initiatefcommunication, the procedure of sending a calling :phrase to the .tractor plane pilot to be 'sounded at llo-ud speaker'LSI, the sendingoi a return acknowledging `phrase to be sounded at loud speaker LS2and1then the subsequent carrying on y.oi the conversation may be similar to that just described when communication was initiated -by 4the vtractor plane pilot.

In Fig. 2, the tractor TP land lglider GL are Aconnected by a transmitting circuit the same as thatfo'f Fig. 1. In Fig. 2, the apparatus mounted on the planes includes telephone equipment lthe same as in Fig. 1, and also control equipment wherewith the tractor plane pilot can automatically establish anyone of a plurality of diierent control conditions on the glider, and wherewith a return indication is effected on the tractor plane to indicate 'that the control Yestablished on the "glider fis in force.

l.Looking at tractor plane TP of Fig. 2, its transmitter includes two oscillators OSI and OS2 in addition to the microphone MI, modulator-ampliier MAI and output transformer TI. These oscillators OSI and OS2 may be of any suitable type, such as an electron tube oscillator and are shown conventionally for the sake of simplicity. It is sufficient for this application to point out that oscillators OSI and OS2 supply current of a preselected frequency, and |by way of illustration I shall assume that oscillators OSI and OS2 supply current of 40 and 50 cycles per second, respectively. Oscillators OSI and OS2 and microphone MI are selectively connected to the input side of modulatr-amplifler MAI through a manually operable switch SW3, which has three positions. With switch SW3 set at its left-hand position, that is, the position shown by the solid line in Fig. 2, oscillator OSI is connected to the modulator-amplier and the electromotive force thus induced in secondary winding 8 of the output transformer TI is a carrier modulated at 40 cycles per second. With switch SW3 set at its mid position, the oscillator OS2 is included in the input circuit to the modulator-amplifier MAI and the electromotive force appearing in secondary winding 8 of the output transformer is the carrier modulated at 50 cycles per second, and with the switch SW3 set at its right-hand position, the microphone M2 is included in the input circuit of the modulator-amplifier and a carrier telephone` current appears in the output of the transformer TI.

The receiver of the tractor plane of Fig. 2 is the same as that of Fig. 1, with an indication means added. This indication means includes a tuned circuit comprising a condenser 50 and an inductance 5I connected in series across the output side of the demodulator-amplier DAI, t0- gether with a relay 52 and an indicator lamp 54. This indication circuit 50-5I is tuned to resonance at some preselected frequency, such as, for example, 30 cycles per second, and the indication relay 52 is connected through a rectier 53 to at least a portion of the inductance 5| and when energized the relay -52 controls a simple circuit for lamp 54.

The switching means for the apparatus of the tractor plane of Fig. 2, includes in addition to switches SWI and SW3, a coder CD, a push button PBI and another switch 55. The type of coder CD is immaterial, and a relay' type is shown, the arrangement being such that as long as the winding of this coder is supplied with current by a source not shown, its contact member 56 is operated to alternately engage contacts 51 and 58 at a preselected code rate, such as, 75 times per minute. With the switch SWI set at its normal position closing contact 26, the coder CD alternately connects the transmitter and receiver of the apparatus of the plane TP to the terminals 9 and I3 of the transmitting circuit, secondary winding 8 of output transformer TI being connected to terminals 9 and I3 through coder contact 56-51, and the input side of the demodulator-amplier DAI being connected across the terminals 9 and I3 through coder contact 56-58. When switch SWI is moved to its dotted line position, closing contact II, then the transmitter and receiver are connected to the terminals 8 and I3 through the contacts of push button PBI, the secondary winding 8 of transformer TI being connected to the terminals 9 and I3 through the normally open contact 60 of push button PBI and the input side of the demodulator-amplifier DAI being connected to the terminals 9 and I3 through the normally closed contact 59 of the push button.

The switch 55 is inserted in the connection of the loud speaker to the output side of the demodulator-ampliiier, and it may be used to at times cut out the loud speaker.

The transmitter of glider GL of Fig. 2 includes an oscillator OS3 in addition to microphone M2, modulator-amplifier MAZ and output transformer T2. Oscillator OS3 is connected to the input side of the modulator through the normally closed left-hand position of a switch SW4 and the microphone M2 is connected to the modulator through the normally open right-hand position of switch SW4. Oscillator OS3 is an indication oscillator and on the assumption that the indication means of the tractor plane TP is tuned to resonance at 30 cycles per second, the oscillator OS3 supplies current of 30 cycles per second. Consequently, when switch SW4 is set at its left-hand position the oscillator OS3 is connected to the modulator and an indication current modulated at 30 cycles per second appears in the secondary winding 34 of the output transformer T2. With switch SW4 moved to its right-hand position, then a carrier telephone current is made to appear in the secondary winding 34 of the output transformer.

The receiver of the glider of Fig. 2 includes two control circuits and associated control relays in addition to the demodulator-amplifier DAZ and loud speaker LSZ. One control circuit comprises a condenser 60 and an inductance 6I, across at least a portion of which inductance a relay LI is connected through a rectier 62. Similarly a second control circuit includes a condenser 63 and an inductance 64, across at least a portion of which inductance a relay L2 isl connected through a rectifier 85. These control circuits are connected in multiple across the output of the demodulator and are tuned to the frequencies of the oscillators OSI an OS2 of the transmitter of the tractor plane, the circuit including condenser 60 and inductance 5| being tuned to the frequency of 40 cycles per second and the circuit including condenser 63 and inductance 84 being tuned to resonance at 50 cycles per second. Relays LI and L2 are used to govern diierent control devices, such as different positions of an automatic pilot of the glider plane.

The switching means of the glider of Fig. 2 includes two relays 1I and 12, a push button P132 and another switch 13. This switching means will be best understood from a description of the operation of the apparatus.

Assuming the apparatus of Fig. 2 is set for control condition, the two switches SWI and SW2 are placed so as to close contacts 26 and 42, respectively, and switches SW3 and SW4 are set at their left-hand positions, that is in the positions shown by the solid lines in the drawing. Under these circumstances, a control carrier current modulated at 40 cycles per second due to oscillator OSI is impressed upon the terminals 9 and I3 of the transmitting circuit during the code period that contaclt 56-51 of coder CD is closed. A corresponding electromotive force appears at terminals 35 and 39 of the glider and such electromotive force is applied to the input side of the demodulator DA2 through a connection extending from terminal 35, wire 38, contact 42 of switch SW2, wire 80, back contacts 19 and 8| of relays 1I and 12, respectively, top righthand terminal to lower right-hand terminal of demodulator DAZ and wire 83 to terminal 39. After demodulation a control current of 40 cycles is applied to the output side of the demodulator to energize relay LII through. its corresponding circuit, andl with relay: LI! energized to close its front contact 14, al preselected control circuit is closed. Loud speakerLS2 is also energized by this current 11o-soundy a corresponding tone unless fthe switch 13 is placed at its open position. At the end of the code period of coder CD and with contact 56-51 open, this control current is terminated, with the result that relay LI is released; During the period relay LI is picked up. closing, front. contact. 16s a. simple circuit, is fonmed for energizingrelay 1.I and thait relay on picking up closes iront contact 1'1' andi relay.12 is energizedV by an obvious. circuit including front contact 11-. When relay LI is released at ther end ofV this code period, relay 1I isimmediat'elzy released andrelay 12 is deenergized; but'k relay 1-2 remainsl pickedY up for a period due toy its; slow release characteristic. During the period relay 1:I- is released and relay 12 ispicked up; the, sec,- ondary winding 34 of output transformer T2k is connected across terminals 35 and 39, the connection to terminal 39 being completed through wires 40 and 83, and to terminal 35 throughI wire` 35, front c'fontact 18foflrelay'12, back contact 19 of relay 1I, wire 80, Contact 42 of switch SW2 and wire 38. Thus during this slow release period of relay 12, indication current is impressed across terminals 35 and 39 since oscillator OS3 is normally connected to the input side of the modulator-amplifier MA2. Coder CD has now closed contact 56-58 to connect the input side of demodulator-amplifier DAI across terminals 9 and I3, and the indication current is supplied to the demodulator and current of 30 cycles per second appears in the ouput side to energize indication relay 52 and to cause the illumination of lamp 54. A corresponding tone would also be sounded -at loud speaker LSI unless the switch 55 has been moved to its open position.

In the event the switch SW3 is moved to its mid position so that oscillator OS2 is connected to the input side of the modulator MAI, a carrier current modulated at 50 cycles per second is applied to the transmitting circuit during the period coder contact 56-51 is closed, with the result the relay L2 of the glider is picked up to effect a corresponding control. With relay L2 picked up to close front contact 82, the switching relays 1I and 12 are energized in the manner explained when relay LI is picked up with the result indication current is supplied to the transmitting circuit during the slow release period of relay 12, and since coder CD has operated ,to close the connection olf the input side of the demodulator to the transmitting circuit, this indication current will bring about the illumination of lamp 54, the same as in the previous case.

In the case the tractor plane pilot wishes to telephone to the glider pilot, the tractor plane pilot would shift switch SW3 to its right-hand position to connect microphone MI to the modulator, shift the switch SWI to close contact II, depress push button FBI to close contact 3B, and then speak a preselected calling phrase into the microphone MI. The discontinuing of the control current would cause the release of both relays 1I and 12 on the glider which in turn would connect the input side of the demodulator across terminals 35 and 39, the connection being completed through back contacts 19 and BI of relays 1I and 12, respectively. The carrier telephone current would be applied to demodu- 10 lator DAZ and the. calling: phrase would" beA reproduced at loud speaker L82. The glider. pilot would close switch 13 when the. control. current ceasestobe received if switch] 3 is not; normally closed; The glider pilot. upon. receiving. such: a calling.' phrase would'. shift switch. SW2 to close contactl andl would. shift switch SW4: to its right-hand: position to.- connect the' microphone M2 to the m'odulatorand would. then. speak the designatedanswering phrase into4 his loud speakeran rlhe. corresponding carrier telephone cur--- rent would: be appliede (to thef transmitting circuit. The tractor pl-ane pilot. would release in the meantime. the push buttonPBI so that the input sidek off demodulator'L DAI: is connected: across; terminals` 9.A and lf3,A andthe answering phrase would be sounded at loudspeaker LSI. Frorn` this. poinlt on the twoy pilots would carrir` onptheir conversation by properoperation'of the push buttons PBI.: andr FB2. At the end orf` the telephone. conversation. each pilot. would restore hisswitching means to.- its; normal position so. thatlthe'controlcondition wouldbe reestablished;

In the event. the control" condition is in effect and the glider pilotwishes to initiate telephone communica-tion, he-wouldv first `shift switch SWL to its right-hand position and discontinue the sending of indication current which would result in the release of the indication relay 52 and the extinguishing of the indication lamp 54 of the tractor plane. The extinguishing of lamp 54 would be an indication to the pilot of the tractor plane that he could expect telephone communication, and he would move his switch SWI to close contact II. Under such condition the telephone current created by the glider pilot speaking the calling phrase into microphone M2 would be applied to the demodulator DAI on the tractor plane and reproduced at loud speaker LSI. After such calling phrase the tractor plane pilot would speak the designated acknowledging phrase into microphone MI. From this point the conversation would be carried on between the two pilots by the proper operation of the push buttons PBI and FB2. It is to be observed that if in Fig. 2 a control condition only is required, then the telephone apparatus would be omitted. Also, if more than two control conditions are required, such can be provided by including additional oscillators in the transmitter at the tractor plane and including additional corresponding tuned circuits in the output of the demodulator of the glider.

It is to be noted that in Fig. 2, the first stage tubes VI and V2 have been omitted but such tubes may be used lf desired.

Also, it is to be understoodl that the frequencies recited for the carrier and for the control and indication oscillators are for illustration and other frequencies can be used.

Although I have herein shown and described but two forms of communication and control systems for air planes embodying my invention, it is understood that various changes and modications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination, a motorless plane attached to a motor plane by a towing cable, said cable formed of a non-conductive elastic material which permits stretching of the order of thirty per cent of the normal length of the cable to tion of the plane on which the equipment is lop cated and to said cable to electrically associate said equipments through a circuit including said cable and the capacitance between said metallic portions of the planes.

2. In combination, two air fplanes connected by a towing cable formed of non-conductive elastic material which permits a stretching of the order of one-third of its length, a conducting compound applied to said cable to impart a predetermined conductivity thereto, means to insulate the connection of said cable to each plane, an electrical apparatus including a source of oscillatory current and a receiver of such current located on each plane, and each of said electrical apparatuses connected to said towing cable and to a metal portion of the plane on which the ap- 25 paratus is located to associate the two apparatuses due to the conductivity thus imparted to said towing cable and the electrostatic coupling between the metal portions of the planes.

ANDREW J. SORENSEN.

REFERENCES CITED The following references are of record in the le of this patent:

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